The invalid-params error paths already listed available agents,
but the unknown-agent path in runSingleAgent just returned
'Unknown agent: claude' with no hint what exists.
Now: 'Unknown agent: "claude". Available agents: "worker".'
Model would guess names like "claude", "default", or skill names
like "brave-search". Now it self-corrects on the next call.
Tested with Opus 4.6: without fix, model gave up on subagent
and ran the tool calls itself instead. With fix, 1 wasted call.
dae2eb5bfa